docs: def _keplerian_to_keplerian_circular(cls, coord, center):
          """Conversion from Mean Keplerian to Keplerian near-circular elements
          """
          a, e, i, Ω, ω, ν = coord
  
          ex = e * cos(ω)
          ey = e * sin(ω)
          u = ω + ν
  
          return np.array([a, ex, ey, i, Ω, u], dtype=float)